1. Field of the Invention
The present invention relates to a picture coding apparatus which is intended for high efficiency coding of digital video signals.
2. Description of the Prior Art
FIG. 1 is a block diagram showing the constitution of a picture coding apparatus in the prior art operating at a definite transmission rate, such as disclosed in Murakami, Asai et al. Interframe Coding using "Adaptive Type Vector Quantizer" in Technical Report IE84-1 of the Institute of Electronic and Communication Engineers in Japan (Apr. 3, 1984). In FIG. 1, the apparatus comprises a frame skip processor 1 for performing a frame skip processing to an input picture data 101 per each frame:a subtractor 11 for performing subtraction between a remaining input picture data 102 after the frame skip processing and an interframe prediction signal 107; a quantizer 2 for blocking an interframe differential signal 103 output from the subtractor 11 and then quantizing the block [hereinafter referred to as "quantization block (QB)"] using prescribed quantization characteristics assigned by a quantization characteristics control signal 112 and outputting a quantization index 104; a quantization decoder 3 for decoding the quantization index 104 using the quantization characteristics control signal 112 and outputting a decoded differential signal 105; an adder 12:a frame memory 4 for generating the interframe prediction signal 107; a variable word length coder 5; a transmission buffer 6 for outputting a transmission picture data 113; and a conventional coding controller 7b for controlling the frame skip in the frame skip processor 1 according to a buffer occupancy 108 of the transmission buffer 6 and control of the quantization characteristics in the quantizer 2.
Next, operation will be described. First, in the frame skip processor 1, frame skip processing is performed to input picture data 101, per each frame, according to a frame skip control signal 111 assigning the number of frame skip. A remaining input picture data 102 outputted from the frame skip processor 1 is subjected to subtraction of an interframe prediction signal 107 in the subtractor 11 at the rear stage. As a result, an interframe differential signal 103, reduced in temporal redundancy, is generated. In the quantizer 2, the interframe differential signal 103 is divided into quantization blocks QB, and every n.times.QB (n: natural number) (every prescribed quantization blocks) or every frame, quantization of the quantization blocks QB is performed using prescribed quantization characteristics assigned among a plurality of quantization characteristics using a quantization characteristics control signal 112. The conventional coding controller 7b outputs a frame skip control signal 111 and a quantization characteristics (quantization step size) control signal 112 according to a buffer occupancy 108 of the transmission buffer 6. FIG. 2 is a block constitution diagram of the conventional coding controller 7b. Reference numeral 81 indicates a third quantization characteristic control table. The relationship of the buffer occupancy 108 and the quantization characteristics (quantization step size) G and the number of the frame skip M is shown as follows: EQU B:small.fwdarw.G and M:small EQU (.fwdarw.number of bits created by coding I:increase) EQU B:large.fwdarw.G and M:large EQU (.fwdarw.number of bits created by coding I:decrease).
The quantization index 104 and the quantization characteristics control signal 112 are coded and multiplexed in the variable word length coder 5, and transmitted to the transmission buffer 6, and temporarily stored. They are and then transmitted as transmission picture data 113. In the quantization decoder 3, the quantization index 104 is decoded using the quantization characteristics control signal 112, and as a result of decoding the quantization index 104, a decoded differential signal 105 is outputted to the adder 12. The interframe prediction signal 107 is added to the decoded differential signal 105 by the adder 12, and a decoded video signal 106 is generated. In the frame memory 4, the decoded video signal 106 is subjected to the frame delay and outputted as the interframe prediction signal 107.
Since the picture coding apparatus in the prior art is constituted as described above, the interframe differential signal is divided into the quantization blocks QB, and the decoding control every n.times.QB or every frame is performed, according to the buffer occupancy. Consequently, problems exist in that when the control is performed every n.times.QB, the picture quality may become ununiform, locally, resulting in deterioration of the subjective quality. Further, when the control is performed every frame, since the coding control is delayed, a larger step size may be selected or the frame skip may occur frequently.
In other words, the coding control every frame is advantageous in that relatively uniform quality is obtained in the coded picture and the control is performed to the fluctuation for a long period of the number of bits created by coding the picture depending on the content of the scene. However, it is disadvantageous in that as the number of bits created by coding increases, the jerky picture frame is liable to occur accompanying with the increase and the significant fluctuation of the number of frame skips. On the other hand, in the coding control of every block (n.times.QB) where one picture frame is blocked (for example, 4 pixels by 4 lines), advantages exist in that the number of frame skips is relatively constant and the control is performed to the fluctuation for a short period of the number of bits created by coding the picture in each portion of the scene. However, disadvantages exist in that the coding distortion is concentrated to a specific portion on the picture frame and an unnatural picture quality is liable to occur.